<style>
    /* 最顶部导航栏 */
.topnav {
/* margin-top: 20px; */
padding-top: 20px;
color:#000;
font-size:20px;
}
.logo{
font-size: 28px;
font-family: 'Times New Roman', Times, serif;
}
.topnav_ri {
float:right;
}
.topnav_ri a {
color:#000;
margin-left:20px;
}
.topnav_ri a:hover{
color: #1f8bc9;
}
.container {
width: 1200px;
margin: 0 auto;
}
.reg_login{
border-radius: 5px;
width: 50px;
padding: auto;
background: #000;
margin-left: 20px;
}
.reg_login a {
text-align: center;
color: #fff;;
padding: 5px 5px 5px; 
margin-left: 5px;
}

  </style>
 <header class="topnav">
    <nav class="container">
       <span class="logo">First Blog</span> 
        <span class="topnav_ri">
            <a href="/index.html">首页</a>
            <a href="#">归档</a>
            <a href="/about.html">关于</a>
            <span class="reg_login" >
                <a href="#">登录</a><a href="#">注册</a>
            </span>
        </span>
    </nav>
</header>